1. Field of the Invention
The present invention is directed generally to a method of manufacturing discharge lamps, and a discharge lamp including a halogen-introduction carrier.
2. Description of Related Art
High-pressure mercury lamps having a very high mercury vapor pressure when lighted, such as 200 bar (approximately 197 atmospheres), have been used recently as discharge lamps for such applications as backlight for projection-type liquid crystal display equipment instead of metal halide lamps. (See, U.S. Pat. No. 5,109,181 and U.S. Pat. No. 5,497,049.)
U.S. Pat. No. 5,109,181 is directed to a high-pressure mercury lamp including tungsten discharge electrodes mounted in a discharge vessel within which are sealed a rare gas, such as mercury in the amount of at least 0.2 mg/mm3, and a halogen in the amount of 1xc3x9710xe2x88x926 to 1xc3x9710xe2x88x924 xcexcmol/mm3, and which operates with a tube wall load of 1 W/mm2. In this high-pressure mercury lamp, the reason for including a halogen is to prevent darkening of the tube wall of the discharge vessel. A method of introducing the halogen bromine in the form of methylene bromide (CH2Br2) is described as the method for incorporation of the halogen in the high-pressure mercury lamp.
When halogen is sealed into the discharge vessel in the form of methylene bromide, however, carbon and hydrogen are also introduced into the discharge vessel. Accordingly, if the amount of methylene bromide introduced is increased in order to incorporate the large amount of halogen needed in the resultant high-pressure mercury lamp, larger amounts of carbon and hydrogen will be introduced as well. As a result, the resulting high-pressure mercury lamp is liable to phenomena such as loss of transparency due to darkening or frosting of the tube wall of the discharge vessel arising from the carbon and hydrogen.
In manufacturing the high-pressure mercury lamp described above, after the airtight seal portion on one side of the quartz glass tube that serves as the discharge vessel is formed, methylene bromide is introduced through a forked gas introduction tube, after which the other air-tight seal portion is formed. During the manufacture of that high-pressure mercury lamp, accurate control of the amount of halogen incorporated is very difficult, since the methylene bromide is a gas and it is necessary to control the working temperature.
Another known means of incorporating the halogen is to introduce a metal halide in the form of a pellet. This method is conventionally used in the manufacture of metal halide lamps. In very high-pressure mercury lamps that have been used as light sources for liquid crystal projectors in recent years, the volume of the discharge space formed by the discharge vessel has been small, no more than 80 mm3, in order to raise the working voltage when lighted; the amount of halogen necessary to prevent blacking of the tube wall of the discharge vessel in this case is 3 xcexcg or less in weight. Because even the smallest of the generally available pellets are 20 xcexcg in weight, an excessive amount of halogen is incorporated into the discharge vessel. As a result, the halogen causes wastage of the discharge electrodes, and the undesirable phenomenon of light-spot shift.
The technology of introducing mercury halide into the discharge vessel by means of vapor deposition on some structural elements of the lamp has been presented in published European Patent Application EP 0949657 as another method of incorporating halogen. More specifically, halogen is introduced into the discharge vessel via vapor deposition of mercury halide on the metal electrodes. Compared with the two methods described above, this method is superior in that it is possible to incorporate a small amount of halogen without introducing hydrogen and carbon as well. However, it is difficult to control the amount of halogen applied by vapor deposition, and thus, there is wide variation of the amount of halogen incorporated. In the conventional methods, as stated above, it is difficult to accurately introduce a very small amount of halogen without causing damage to the discharge vessel.
The present invention has been developed to overcome at least the aforementioned prior art problems. Thus, an object of the present invention is to provide a manufacturing method for discharge lamps that is capable of regulating accurately very small amounts of halogen, unaccompanied by carbon and hydrogen in a discharge vessel, and that is capable of introducing the halogen using a very easy operation.
A second object of the present invention is to provide a discharge lamp that has an adequate working pressure, and maintains good lighting conditions for extending periods in comparison to conventional devices.
A third object of the present invention is to provide a halogen-introduction carrier that can handle accurate amounts of metal halides even in minute quantities.
These and other objects are solved by providing a method of manufacturing discharge lamps in which a halogen is sealed in a discharge vessel including a main tube that forms a light emitting discharge space and auxiliary tubes connected thereto, and whereby halogen is introduced into the discharge vessel by heating a halogen-introduction carrier including a porous body containing metal halides. In the aforementioned method of manufacturing discharge lamps, the halogen-introduction carrier is preferably positioned inside the discharge vessel and can be heated from an outside source. Preferably, it is desirable that the halogen-introduction carrier is reusable after releasing all of the metal halides. It is also desirable that the metal halide be composed of at least one of bromine, chlorine, iodine and mercury, and that the halogen-introduction carrier be a porous body of tungsten.
The discharge lamp of the present invention includes a light emitting discharge space having a volume which is preferably no greater than 80 mm3 and the predetermined amount of halogen is between 1.7xc3x9710xe2x88x924 xcexcmol/mm3 and 6.7xc3x9710xe2x88x924 xcexcmol/mm3. In addition, in the discharge lamp in accordance with the present invention, halogen is sealed in the discharge vessel, and the halogen-introduction carrier is located within the discharge vessel. The halogen-introduction carrier of the present invention can be a porous tungsten body of which the density of tungsten metal is 40% to 70%.
According to the method of manufacturing discharge lamps described above, the halogen is introduced via a halogen-introduction carrier that is preferably a porous body in which a metal halide is absorbed in a roughly unimolecular layer. For that reason, it is possible to control the target amount of halogen to be introduced, even when the amount is very low, by controlling the amount of the halogen-introduction carrier used for introduction. Moreover, the operation of introducing the predetermined amount of halogen is very easy. And since the halogen is introduced via a metal halide, there is no possibility that carbon or hydrogen will be introduced into the discharge vessel.
In the discharge lamp of the present invention, the desired amount of halogen is sealed into a discharge vessel that has a discharge space of small volume. It is possible, therefore, to obtain accurately the target luminous flux maintenance ratio, and to prevent the occurrence of undesirable phenomena such as light-spot shift due to wastage of the discharge electrodes that arise from an excessive amount of halogen. Advantageously, no hydrogen or carbon is introduced into the discharge vessel, and thus, there is no darkening or loss of transparency of the discharge vessel tube wall.
Preferably, the halogen-introduction carrier of the present invention is a porous body of tungsten, has a relatively great specific surface, and can absorb metal halides in a roughly unimolecular layer. For that reason, it is possible to control the target amount of halogen to be introduced, even when the amount is very slight, by controlling the amount of the halogen-introduction carrier used.